Roundcube is an independent open source project with a developer community of its own providing support, documentation, themes and plugins. Roundcube's feature set offers many of the features mainstream clients provide:

Themes for Roundcube

Themes on Roundcube are called skins. You can find a variety of skins here and here. There is a free/by-donation theme that looks like Google:

You can also purchase skins, typically for US$99. These provide advanced look and feel environments, including mobile web support. A few provide similar look and feel to familiar mainstream environments.

You should see this at your Roundcube login—notice the checkbox at the bottom:

Two Factor Authentication for Roundcube

If you'd like to add two factor authentication for your Roundcube email, check out Alexandre Espinosa Menor's Two Factor Google Authenticator. I won't walk through the installation and configuration here but you can see how it works. The installation instructions are similar to the plugin we installed above.

You should see the Junk icon in your command buttons in the Roundcube client:

Whenever you encounter a junk message, just click the Junk button and the iRedMail server components for spam filtering will be trained.

The Plugin Manager

You can also install the Roundcube Plugin Manager. It makes it possible for individual Roundcube users to install their own plugins.

Plugin Manager emerged with the idea of putting users in control of their webmail preferences allowing them to enable and disable functionality (plugins) per account to fit their personal needs while providing administrators a more simple way to control, manage and update roundcube plugins at the same time.

Installing Plugin Manager is exactly the same as the process we used with the other plugins above. See here for installation instructions.

Note: Using Plugin Manager with some plugins requires that you purchase credits in a virtual currency.

PGP Encryption With Roundcube

PGP encryption is one of the most frequently requested features for Roundcube and for good reasons more and more people start caring about end-to-end encryption in their everyday communication. But unfortunately webmail applications currently can’t fully participate in this game and doing PGP encryption right in web-based applications isn’t a simple task. Although there are ways and even some basic implementations, all of them have their pros and cons. And yet the ultimate solution is still missing.

The only active project listed is Mailvelope which manages encryption in the browser. It's available for Chrome and Firefox.